The present invention relates to battery sets employing a plurality of battery cells in combination, and more particularly to a battery set composed of small sized secondary battery cells and a method for producing electric power output to be suited for use in driving a prime motor of an electric vehicle.
In recent years, a long-standing concern of a background for achieving an environmental protection by reducing carbon dioxide emission is how to promote introduction of clean vehicles such as an electric vehicle (EV) or a hybrid electric vehicle (HEV) in place of automobiles, which use fossil fuels, such as gasoline fueled vehicles in an automobile industry. In order to achieve this end, an extensive research and development work has been undertaken to provide a battery, for driving a prime motor, which has a key factor for a promoted commercialization of such a clean vehicle. In such a battery, it has been a usual practice to employ a secondary battery which is repeatedly rechargeable. In a particular field such as EV or HEV where a large demand exists for an increased power output and an increased energy density for driving the primer motor, it is difficult to manufacture a single unit of a large sized battery and, hence, a plurality of small sized battery cells are generally connected in series to provide a battery set having the large capacity.
With such a structure, however, each unit of the battery cells should have an extremely increased capacity, requiring a specific production line for such battery cell. With the battery set which requires the large capacity for use as the EV""s battery, the weight of a single battery inevitably and remarkably increases, with a resultant difficulty caused in the handling work of the battery.
To address this issue, a large number of small sized battery cells, which are easy to handle, are employed and are connected to provide a battery set suited for use in the EV or HEV. In a case where a LiMn2O4 type lithium ion secondary battery, which has the high power output and high energy density, is used as a battery set for the automobile for charging and discharging characteristics, it has been a usual practice to provide a battery set composed of battery cell groups, each including a plurality of parallel connected unit battery cells, which are connected in series to provide electric power output of 400 V.
However, with such a state-of-the-art battery set, if either one of the battery cells is internally short circuited, the battery set inevitably becomes inoperative such that the EV/HEV vehicle is rendered inoperative.
For this reason, it is required for the state-of-the-art battery set of the EV/HEV vehicle to have respective battery cells to be installed with respective cell controllers to monitor the operating state of each battery cell, thereby increasing the manufacturing cost. In the event that at least one piece of the battery cells essentially encounters an issue, the battery set is adversely affected with such an issue to be rendered inoperative.
The present invention has been-made in view of the above issue in mind and has an object of the present invention to provide a battery set which is enabled to produce a large amount of electric power output at a large capacity, which is able to reduce the number of components parts without the need for increasingly mounting sensors and controllers to circuits which constitute the battery set and which is not adversely affected by the presence of short circuited condition of a single battery cell.
It is another object of the present invention to provide a method for producing electric power output with a battery set which is not adversely affected with a short circuited battery cell and which is specifically suited for use in an electric vehicle or a hybrid electric vehicle.
Considerable research and development work has been undertaken to address the aforementioned issue by a utilizing a difference between a normally available voltage and voltage under an overcharged condition of to a single battery cell such that plural series-connected battery-cell groups are connected in parallel to be combined and by utilizing more than four pieces of LiMn2O4 type lithium ion battery cells, with a resultant success performed in the present invention.
According to a first aspect of the present invention, there is provided a battery set which comprises a plurality of groups of series-connected battery cells, wherein each group includes a voltage of x [V] in a normal state and a voltage of y [V] in an overcharged state wherein said each group includes more than y/(yxe2x88x92x) pieces and less than 400/x pieces of battery cells which are connected in series, and wherein said plurality of groups of series-connected battery cells are connected in parallel.
An important feature of the present invention concerns the battery cell groups, each of which includes more than four pieces of LiMn2O4 type lithium ion battery cells connected in series, and which are connected in parallel.
With such an arrangement, the battery set of the present invention includes more number of pieces of unit battery cells which constitute the series-connected battery-cell groups than those used in the state-of-the-art battery set. In effect, in a case where an internal resistance of the battery cell unlimitedly reaches the zero value due to internal short-circuited state of the unit battery cell, electric current tends to flow so as to equalize the voltage between a particular battery-cell group, inclusive of the short-circuited unit battery cell, and the other remaining battery-cell groups, without rendering the other remaining battery-cell groups to be over-charged to allow the battery set to be temporarily available for use in a continuous fashion.
According to a second aspect of the present invention, there is provided a method for producing electric power output, which comprises preparing a plurality of groups of series-connected battery cells wherein each group produces a voltage of x [V] in a normal state and a voltage of y [V] in an overcharged state and wherein each group includes more than y/(yxe2x88x92x) pieces and less than 400/x pieces of battery cells which are connected in series, and connecting the plurality of groups of the series-connected battery cells in parallel to form a battery set to obtain electric power output.